Mel- Each disc on the harp is screwed onto a spindle-a steel bar that runs between the two action plates. Each spindle has a brass action arm pressed onto knurling on its shaft and the linkage is riveted to the action arm.
The chain of movement in a pedal harp when you move a pedal is this: When you press your foot on the brass pedal, that moves the steel pedal bar inside the pedestal, which moves the pedal rod that runs up the column, which moves the main action gears at the column end of the action. Those gears divide the pedal rod movement between the natural and sharp chains, i.e., the linkage. The linkage moves the brass action arm which moves the spindle which moves the disc.
Each spindle has a disc screwed onto it on the string side of the neck, and has a hollow back plate screw with a little spring in it pressing on it from the back plate side of the instrument. The head of the spindle is cone shaped and so the back plate screw presses the cone shaped spindle head into a cone shaped hole in the front plate. If the back plate screw is not pressing the spindle hard enough against the front plate, the spindle will chatter back and forth between the front and back plates when the string is plucked(and the disc engaged). If it is pressing the spindle too hard against the front plate, it can bind up the chain so it moves hard or not at all. So the trick is to adjust the back plate screw so it pushes the spindle head against the front plate hard enough to not chatter, but not so hard that it binds up the movement.
Back plate screws have to be tighter(meaning pressing harder against the back end of the spindle) at the bass end of the action then higher up because the string tension is higher and the swing of the vibrating string is greater than higher on the instrument.
When you press your thumb against the prongs of a disc, you’re checking to see how hard the back plate screw is pressing on the spindle. If you can’t press the spindle back at all, then the back plate screw is too tight and should be backed off a little.
I’m sure that’s far more than you ever wanted to know about your harp action!
.
